A separator is an essential component for fuel cells along with a membrane electrode assembly (MEA) and performs various functions, such as structural support for the MEA and gas diffusion layers (GDLs), collection and transmission of current, transmission and removal of reaction gas and reaction product, transmission of water coolant used for removing reaction heat, and the like.
Hence, it is necessary for separator materials to have excellent electrical and thermal conductivity, air-tightness, chemical stability, and the like.
Generally, graphite-based materials and composite graphite materials consisting of a resin and graphite mixture are employed as the separator material.
However, graphite-based materials exhibit lower strength and air-tightness than metallic materials, and suffer from higher manufacturing costs and lower productivity when applied to manufacture of separators. Recently, metallic separators have been actively investigated to overcome such problems of the graphite-base separator.
When the separator is made of a metallic material, there are many advantages in that volume and weight reduction of fuel cell stacks can be accomplished via thickness reduction of the separator, and in that the separator can be produced by stamping and the like, which facilitates mass production of the separators.
In this case, however, the metallic material is likely to undergo corrosion during the use of the fuel cell, causing contamination of the MEA and performance deterioration of the fuel cell stacks, and a thick oxide film can form on the surface of the metallic material by extended use of the fuel cells, causing an increase in internal resistance of the fuel cell.
Stainless steel, titanium alloys, aluminum alloys, nickel alloys, and the like are proposed as candidate materials for the separator of the fuel cell. Among these materials, stainless steel has received attentions for its lower price and good corrosion resistance, but further improvements in corrosion resistance and electrical conductivity are still needed.
Referring to FIG. 1, Japanese Patent Laid-open Publication No. 2003-277133 discloses a technique that distributes relatively inexpensive carbon powder 3 in a passive film 2 on a metallic separator to improve electrical conductivity of the metallic separator.
However, when applying such fuel cells of the disclosure to a vehicle, the carbon powders 3 tend to be separated due to vibration and the like generated when driving the vehicle, and, stainless steel 1 of the metallic separator exhibits a high contact resistance without proper pretreatment, making it difficult to use the metallic separator for the fuel cell.
Referring to FIG. 2, Japanese Patent Laid-open Publication No. 2000-353531 discloses a technique that forms a titanium nitride film 2 on stainless steel 1 through high temperature nitridation of titanium.
However, since this process requires a long period of treatment and must be performed under vacuum, it has difficulty in constitution of a process for mass production and suffers from high manufacturing costs.